u should know better abf, tsk tsk....
depends on what u qualify as "better performance for the buck".
if u want to buy a low end $600 notebook w/ 256MB of RAM, 2 hours of battery life or less, an overly flexible chassis, a lack of firewire and bluetooth, and integrated video for your absolute basics then yes, get the acer and get your bang for your buck. if however u want to fill in the blanks of the mentioned missing features on any machines, PC or Mac, start pumping up the price. when u get to a machine that suits the mentioned needs, u will likely be well beyond the price of a 12" iBook (i.e.-stock 12" acer is lighter than the iBook w/o it's external optical drive and larger battery along, has a larger HD, but lacks dedicated GPU and only gets decent battery life over 3 hours when the larger and heavier battery is used, all for $300+ more than a stock 12" iBook.)
all of that said, if u can wait till january, u may have a good chance of getting a truly new iBook w/ the intel change possibly happening thereabouts.
personally i'd wait unless u absolutely need a system now.